1. Field of the Invention
The process of the invention is distinct from the well established processes for producing partially oriented yarn (POY) at lower spinning speeds, for example in the range 3,000 to 4,500 meters/minute. Such (POY) yarns have too high an extension for direct use in a fabric and the yarn requires to be drawn in order to reduce its extension. This drawing stage is often combined with a bulking step.
Experiments have demonstrated that as the spinning speed increases above 5 km/min there is a very rapid increase in stress applied to the threadline in the spinning chimney with much of the draw down occurring within a few centimeters resulting in a neck draw ratio of up to 6.0 and the likelihood of breakage. To be able to operate at even higher speeds it is apparent that the maximum stress, maximum strain rate and, hence, "neck draw ratio" need to be reduced.
Attempts at reducing the "neck draw ratio" by the use of a constant temperature heated shroud immediately below the spinneret have resulted in the draw down point or neck being moved by a distance almost exactly the length of the shroud with only a small increase in yarn velocity prior to the `neck` formation.
2. Description of Related Art
In European Patent Application Nos. 244,217 and 245,011; and U.S. Pat. No. 4,687,610 (all to E. I. Du Pont de Neumours and Company) various techniques are described to control the attenuation profiles of a threadline at high spinning speeds. In European Patent Nos. 244,217 and 245,011 there is described a process for preparing polymeric filaments, wherein the freshly extruded filaments enter an enclosed zone that is maintained at super atmospheric pressure by a controlled flow of air at a low positive pressure, and the filaments leave the zone through a constriction, either a venturi or a tube, assisted by the concurrent flow of such air at a controlled high velocity. In this process the extent of "necking down" that would otherwise be normally experienced by the filaments at the high spinning speeds employed is appreciably reduced so that the filaments are oriented more highly and more uniformly (less difference between amorphous sections and crystalline sections).
In U.S. Pat. No. 4,687,610 a somewhat similar process is described in which the threadline, after leaving the spinneret, passes first through an enclosed chamber supplied with a pressurised gas and then through a tube attached to the underside of the chamber. The tube is also supplied with a pressurised gas. In the process, the velocity profile of the spinning filaments increased smoothly to the final take up velocity without sign of any sudden velocity change or "neck" formation. In British Patent No. 1391471 (Hoechst Aktiengesellschaft) there is described a heater for use in the production of spun filaments having a low degree of pre-orientation i.e. POY yarns. The heater comprises two parts, each of which has the shape of a hollow truncated cone, which are attached to each other at their larger circular openings. The lower part is heated while the inside wall of the upper part reflects the heat emitted by the lower part. The spinning threadline is thus subjected to a variation in temperature as it passes through the heater.
In Japanese Patent Nos. 51067-422 (Teijin) there is described a process in which the spinning polyester threadline is passed through a controlled temperature gradient heating atmosphere. The polyester fibre is taken up at a low speed of 2 km/min. In Japanese Patent Nos 59001-713-A and 58203-112-A (both Toray) the spinning threadline is passed through a heated tube immediately below the spinneret. The temperature in the tube is kept at between the melting point of the polymer and 400.degree. C. with the temperature gradually decreasing downwards. The spun fibre is taken up at a speed between 1.5 and 3 km/min. Japanese Patent No. 62250213 A (Teijin) also describes the use of a cylindrical heater immediately below the spinneret, such heater allowing a decreasing temperature distribution profile to be imparted to the freshly spun filaments in a direction parallel to the filaments. Though the patent refers to spinning speeds of 3 km/min or more, a reading of the specification makes it clear that the described process produces POY yarns and that a subsequent drawing stage is required.
The temperature gradient heating environments used in British Patent No. 1391471 and the above Japanese Patents merely serve to control the physical properties of the spun filaments and/or prevent thermal deterioration of the molten polymer. There is no suggestion that the use of these environments could also be used to reduce "neck draw ratio" in a spinning threadline. Indeed in the spinning of POY yarns `necking` does not occur.